1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method, and an image forming program product.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a laser printer, a digital copier, a fax machine, and a multifunction peripheral, an image creating unit typically performs a negative/positive (N/P) process by radiating a light beam to a position where toner is to be applied in correspondence with an input image (hereinafter, “toner application part”) to form an electrostatic latent image, and applying toner to the electrostatic latent image to develop the image. The volume of the applied toner varies depending on the diameter of the beam spot formed as the light beam reaches a scanning target surface, and also depending on developing conditions. Therefore, even if the same image data was input, the output images would vary.
For example, when an image of a vertical line having a width corresponding to one pixel (vertical line of one pixel) is output, even if a light beam were radiated for a period of time corresponding to a line of one pixel, the thickness of the vertical line in the resultant output image would be different from that of the original image. This is because the beam spot diameter and developing conditions vary depending on the model and the lot number of the image forming apparatus, and also depending on each individual machine.
In a case where a main scanning pixel line (vertical) and a sub scanning pixel line (horizontal) are output on the same recording medium, the horizontal line and vertical line in the output image may have different thicknesses depending on the beam spot diameter and developing conditions. Particularly, when a sheet with an image obtained from a copy operation is used as the original for another copy operation, i.e., when a grandchild copy is taken, the above phenomenon becomes significant.
This phenomenon, in which the output image includes a line with a different thickness from that of the original image, does not only appear when black or colored toner is applied to form the lines of the image. The same phenomenon also appears for lines in the background of the recording medium where toner is not applied. That is, the lines in the background may become thinner or thicker than those of the original image.
The above problem does not only arise when forming a monochrome image using black toner, but also when forming a color image using color toner.
Conventionally, there has been proposed a technique in an attempt to solve such a problem. Specifically, a vertical line having a width corresponding to one dot is detected based on a pattern of the multi-value image data, which pattern includes a target pixel in the input image data and pixels adjacent to the target pixel in the main scanning direction. Then, the printed dots of the vertical line are reduced in size, so that the vertical line is made thinner in the formed image (see patent document 1).
In the above conventional technology, the process of determining the pattern of the multi-value two-bit input data is performed with hardware, and a description is given for a case where the input multi-value data is expressed by 2 bits. There is no description of cases where the number of bits exceeds 2 bits (4 values).
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-63134
In the conventional technology disclosed in patent document 1, the lines in the formed image are made thinner. However, this technology needs to be modified for better image quality. That is, in an image output by the electrophotographic image forming process, the vertical lines do not only become thicker, but may also become thinner than those of the original image. The above conventional technology is not capable of thickening the vertical lines to solve the problem where the lines become thinner than those of the original image.
Furthermore, in addition to determining whether the lines in the formed image become thicker or thinner than those of the original image, it is necessary to determine whether the vertical lines need to be made thinner or thicker in consideration of the relationship between the widths of the vertical lines and the horizontal lines. However, the conventional technology is not capable of flexibly addressing such a relationship. Moreover, the conventional technology only addresses vertical lines, and does not address diagonal lines.
Furthermore, the conventional technology disclosed in patent document 1 indicates that the input image data is expressed by multi-value data. However, the image pattern is determined based on only 2 bits (4 values). For this reason, if the technology were applied to image data with a larger number of bits, the hardware configuration would be extremely extensive. Accordingly, such a configuration would be complex and expensive.